Glue your fabric to two sides of the holder.
Start shoving it full with batting. Glue one more side down and shove as much batting in as you can. You want it to be tight with batting so that you can shove pins in.
Tuck the last side in and glue it down.
Cut out any shape or design out of your felt and then hand stitch it into place.
Stick your pins in and now you have them nice and handy. Simple.